This invention relates to an antenna for operation at frequencies in excess of 200 MHz, and to a radio communication system including the antenna.
The applicant has disclosed a family of dielectrically-loaded antennas in a number of co-pending patent applications. Common features of the disclosed antennas include a solid cylindrical ceramic core of high relative dielectric constant, a coaxial feeder passing through the core on its axis to a termination at a distal end, a conductive balun sleeve plated on a proximal portion of the core to create an at least approximately balanced feeder termination at the distal end, and a plurality of elongate helical conductor elements plated on the cylindrical surface of the core and extending between, on the one hand, radial connections with the feeder termination on the distal end face, and, on the other hand, the rim of the sleeve.
In one of the co-pending applications, GB-A-2292638, there is disclosed a quadrifilar backfire antenna having four co-extensive helical elements formed as two pairs, the electrical length of the elements of one pair being different from the electrical lengths of the elements of the other pair. This structure has the effect of creating orthogonally phased currents at an operating frequency of, for example, 1575 MHz with the result that the antenna has a cardioid radiation pattern for circularly polarised signals such as those transmitted by the satellites in the GPS (global positional system) satellite constellation.
In GB-A-2309592, the antenna has a single pair of diametrically opposed helical elements forming a twisted loop yielding a radiation pattern which is ommnidirectional with the exception of a null centred on a null axis extending perpendicularly to the cylinder axis of the antenna. This antenna is particularly suitable for use in a portable telephone, and can be dimensioned to have loop resonances at frequencies respectively within the European GSM band (890 to 960 MHz) and the DCS band (1710 to 1880 MHz), for example. Other relevant bands include the American AMPS (842 to 894 MHz) and PCN (1850 to 1990 MHz) bands.
GB-A-2311675 discloses the use of an antenna having the same general structure as that disclosed in GB-A-2292638 in a dual service system such as a combined GPS and mobile telephone system, the antenna being used for GPS reception when resonant in a quadrifilar (circularly polarised) mode, and for telephone signals when resonant in a single-ended (linearly polarised) mode.
The applicants have found that, by manipulating the diameter of the conductive sleeve encircling the proximal portion of the core, it is possible to produce a resonance which is characterised by a standing wave around the sleeve rim (referred to herein as a xe2x80x9cring resonancexe2x80x9d) and which occurs at one of the frequencies used in, for instance, mobile telephones or satellite positioning receivers. The ring resonance is effectively a resonance associated with a circular guide mode or ring mode.
According to a first aspect of the present invention, there is provided an antenna having an operating frequency in excess of 200 MHz, comprising a cylindrical insulative body having a central axis and formed of a solid material which has a relative dielectric constant greater than 5, the outer surface of the body defining a volume the major part of which is occupied by the solid material, a conductive sleeve on the cylindrical surface of the insulative body, a conductive layer on a surface of the body which extends transversely of the axis, the conductive sleeve and layer together forming an open-ended cavity substantially filled with the solid material, and a feeder structure associated with the cavity, wherein the said relative dielectric constant and the dimensions of the cavity are adapted such that the electrical length of its circumference at the open end is substantially equal to a whole number (1, 2, 3, . . . ) of guide wavelengths around the said circumference corresponding to the said operating frequency.
One of the difficulties associated with the known dielectrically loaded quadrifilar backfire antenna referred to above is that the bandwidth of the antenna for circularly polarised signals is relatively narrow. This means that manufacturing tolerances tend to be tight, and the antenna may need to be individually tuned to a required frequency. In an antenna in accordance with the present invention it is possible to arrange for the feeder structure to excite a rotary standing wave around the rim of the cavity at its open end, so as to produce an antenna which is resonant for circularly polarised waves and which has an associated cardioid radiation pattern suitable for receiving signals from satellites when used with its axis vertical. The applicants have found that the bandwidth associated with such a resonance is much wider than the bandwidth of the quadrifilar antenna.
It should be noted that the term xe2x80x9cexcitexe2x80x9d is used in this context as a reference to not only use of the antenna for transmitting signals, but also use of the antenna for receiving signals, since the functional characteristics of the antenna such as its frequency response, radiation pattern, etc. obey the reciprocity rule with respect to corresponding transmitting and receiving characteristics. Similarly, references to elements or parts which xe2x80x9cradiatexe2x80x9d when used in the context of an antenna for receiving signals should be construed to include elements or parts which absorb energy from the surrounding space but which, by virtue of the reciprocity rule, would radiate if the antenna were to be used for transmission.
One way of exciting circular standing waves in the sleeve is to employ elongate helical or spiral elements on the surface of the insulative body. In effect, the helical elements impart a tangential component of excitation at the sleeve or sleeve rim so that they may be regarded as tangential excitation or feed means. With appropriate choice of dielectric constant and dimensioning of the sleeve and the helical or spiral elements, the antenna can be made to operate as a dual-mode antenna, with a circular polarisation mode associated with the ring resonance, i.e. a standing wave around the rim of the cavity, and a linear mode associated with the loop resonance referred to above in connection with the twisted loop configuration.
Preferably, at the frequency of the ring mode resonance, the helical elements each have an electrical length equal to nxcexg/4 wherein n is a whole number (1, 2, 3, . . . ) and xcexg is the guide wavelength along the elements at the frequency of the ring resonance.
In this connection, it will be appreciated by those skilled in the art that xe2x80x9cguide wavelengthxe2x80x9d means the distance represented by a complete wave cycle at the frequency in question along the path used for measurement, i.e. the path along which the wave is guided. In the present case, the measurement path is the respective helical element or the sleeve rim, and the guide wavelength is less than the corresponding wavelength in space by a factor which is governed by the dielectric constant of the core material and by the geometry of the antenna structure. It is to be understood that, with the dielectric constant of the core material being substantially greater than that of free space, the guide wavelength xcexg around the rim of the sleeve or along the helical elements is much less than the wavelength in free space, but generally not the same in each case. In the case of the rim, the current path is very strongly affected by the dielectric material because the associated fields are largely within the material, whereas the current paths of the helical elements are less strongly affected, being at the boundary between dielectric material and air.
It is possible, then, to produce a multiple-mode antenna suitable particularly, but not exclusively, for circularly polarised signals without using the narrow band quadrifilar structure referred to above. Consequently, a preferred use of the antenna is for portable or mobile equipment such as multiple-band portable or mobile telephones, particularly cellular telephones, or, more particularly, portable or mobile telephones for the Globalstar and Iridium satellite telephone systems, as well as portable telephones or other units having a GPS or GLONASS positioning function, these satellite services being services which employ circularly polarised signals.
According to a second aspect of the invention, there is provided a radio signal receiving and/or transmitting system comprising a radio frequency front end stage constructed to operate at a first signal receiving or transmitting frequency and, coupled to the front end stage, an antenna which comprises: a cylindrical insulative body having a central axis and formed of a solid material with a dielectric constant greater than 5, the outer surface of the body defining a volume the major part of which is occupied by the solid material, a conductive layer on a surface of the body which extends transversely of the axis, the conductive sleeve and layer together forming an open-ended cavity substantially filled with the solid material, and a feeder structure associated with the cavity, wherein the said relative dielectric constant and the dimensions of the cavity are adapted such that the electrical length of the rim of the cavity at its open ends is substantially equal to a whole number (1, 2, 3, . . . ) of guide wavelengths corresponding to the first signal frequency.
The invention also includes, according to a third aspect, a dielectrically-loaded cavity-backed antenna for circularly polarised waves at a required operating frequency in excess of 200 MHz, comprising a cavity with a conductive cylindrical side wall and a conductive bottom wall joined to the side wall, the side wall having a rim defining a cavity opening opposite the bottom wall, a dielectric core substantially filling the cavity and formed of a solid material having a relative dielectric constant greater than 5, and a rotational feed system, characterised in that the said dielectric constant and the dimensions of the cavity are such that the circumference of the rim is substantially equal to a whole number (1, 2, 3, . . . ) of guide wavelengths at the required operating frequency, and wherein the feed system is adapted to excite a waveguide resonance at the rim of the cavity at the required operating frequency, which resonance is characterised by at least one voltage dipole oriented diametrically across the cavity opening and spinning about the central axis of the cavity thereby to produce a circular polarisation radiation pattern which is directed axially outwardly from the opening of the cavity and has a null in the opposite axial direction.
Further preferred features of the antenna and system are set out in the dependent claims appearing at the end of this specification.
The invention will be described below by way of example with reference to the drawings.